In aircraft and industrial gas turbine applications, brush seals are generally used to seal between rotating and static components. Generally, brush seals are assembled with an interference fit to the rotor in order to allow the bristles to wear in to a line-on-line condition vis-a-vis the rotor. In aircraft engine applications, the rotor is typically coated with a hard coating such as chrome carbide. The rotary components in aircraft and industrial gas turbines are generally stiff. Also, the dynamic behavior of the rotary component is generally not affected by the contact between the rotary component and the brush seal, i.e., the sealing contact between the free ends or tips of the brush seal and the rotary component.
Steam turbines, however, contrary to aircraft and industrial gas turbines, are designed and run above the rotor's first bending critical frequency and often run near the second bending critical frequency. It has been found that application of brush seals in steam turbines in accordance with conventional seal design theory, i.e., with contact between the tips of the bristles and the rotary component, causes the brush seals to impart vibrations to the rotor. It is believed that this is caused by sustained rubbing between the brush seals and the rotor, particularly where the rotor has a bow. Friction from the rubbing leads to uneven temperature distribution about the circumference of the rotor, with the high spot on the rotor increasingly becoming hotter. That is, if the rotor has a bow, the proud part of the rotor is preferentially heated by increasing friction-generated heat which, in turn, further increases the bowing of the rotor with resultant increased vibration.
It has also been found that brush seals do not provide thermal relief during start-up of a steam rotor turbine. At start-up, there is very a little steam flow through the turbine and essentially no steam flow past the brush seal. Without flow, the heat generated by rubbing between the brush seal and the rotor is not dissipated, exacerbating the build-up of heat in the rotor at the seal and particularly in the proud parts. It has been discovered that contact of the brush seals with a steam turbine rotor which operates near or above critical bending frequencies generates sufficient heat to affect rotor dynamic behavior in a manner which causes serious deleterious rotor vibrations.
Another phenomena which is believed to affect the dynamic behavior of the rotor caused by contact with the brush seal is the blow-down effect. Blow-down is the radial inward flow of upstream fluid passing through the brush seal, causing the bristles to tend to move inward toward the rotor. The blow-down effect increases bristle contact pressure and rotor heating when there is initial contact between the bristles and the rotor. It reduces or eliminates initial clearance where provided, tending to increase heating. In any effort to alleviate the rubbing heat-generating frictional contact between the brush seal and the rotary component, the phenomena of blow-down must be accommodated.